Oxyfuel Boiler System and Method of Controlling the Same

ABSTRACT

The oxyfuel boiler system comprises: an oxygen generator; a coal mill; a burner to burn pulverized coal; an after-gas port to which oxygen generated at the oxygen generator is supplied; a boiler provided with the burner and the after-gas port on its wall; a flue introducing combustion exhaust gas from the boiler to the outside; a recirculation gas supply pipe having an exhaust gas tapping port in the midway of the flue and supplying recirculation exhaust gas to the coal mill, the burner, and the after gas port; and an oxygen supply pipe supplying oxygen from the oxygen generator to the burner and the after-gas port, wherein the exhaust gas tapping port is disposed downstream of a dry dust-removing apparatus arranged in the flue, and there is provided an oxygen controlling apparatus for making a concentration of oxygen to be supplied to the after-gas port lower than that of oxygen to be supplied to the burner.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial No. 2008-280675, filed on Oct. 31, 2008, the content of which ishereby incorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to an oxyfuel boiler system and a methodof controlling the same.

BACKGROUND OF THE INVENTION

A coal firing power generation system configured with a pulverized coalfiring boiler and a steam turbine electric generator plays a significantrole, given the recent years' price increase of natural gas and the likeresulting from oil supply shortage and increase of natural gas demand.

As a means to greatly reduce CO₂ emissions from the coal firing powergeneration system, oxyfuel boiler systems have been proposed.

Japanese Unexamined Patent Application Publication No. 2007-147162discloses a technology for adjusting oxygen concentration in the totalgas introduced to a boiler main body by controlling recirculation flowrate of combustion exhaust gas so that the heat absorbing amount of theboiler main body becomes the target heat absorbing amount.

However, in Japanese Unexamined Patent Application Publication No.2007-147162, no consideration is given on reduction of fuel NO_(x).

In view of above, an object of the present invention is to provide anoxyfuel boiler system and a method of controlling the same capable offurther reducing the forming amount of the fuel NO_(x).

SUMMARY OF THE INVENTION

The present invention provides an oxyfuel boiler system in which anexhaust gas tapping port is arranged downstream of a dry dust-removingapparatus arranged in a flue, and there is provided an oxygencontrolling apparatus for making a concentration of oxygen to besupplied to an after-gas port lower than that of oxygen to be suppliedto a burner. More specifically, the present invention provides anoxyfuel boiler system comprising: an oxygen generator; a coal mill; aburner burning pulverized coal; an after-gas port to which oxygengenerated at the oxygen generator is supplied; a boiler provided withthe burner and the after-gas port on its wall; a flue introducingcombustion exhaust gas from the boiler to the outside; a recirculationgas supply pipe having an exhaust gas tapping port disposed in themidway of the flue and supplying recirculation exhaust gas to the coalmill, the burner, and the after gas port; and an oxygen supply pipesupplying oxygen from the oxygen generator to the burner and theafter-gas port; wherein the exhaust gas tapping port is disposeddownstream of a dry dust-removing apparatus arranged in the flue, and anoxygen controlling apparatus for making a concentration of oxygen to besupplied to the after-gas port lower than that of oxygen to be suppliedto the burner is provided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates configuration of an oxyfuel boiler system accordingto a first embodiment;

FIG. 2 illustrates configuration of an oxyfuel boiler system accordingto a second embodiment;

FIG. 3 illustrates configuration of an oxyfuel boiler system accordingto a third embodiment;

FIG. 4 illustrates configuration of an oxyfuel boiler system accordingto a fourth embodiment;

FIG. 5 illustrates an example of a calculation result of reductivecombustion zone temperature of a boiler in the first embodiment; and

FIG. 6 illustrates an example of a calculation result of oxidativecombustion zone temperature of a boiler in the first embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Oxyfuel boiler systems according to respective embodiments will bedescribed below referring to the accompanying drawings. However, thepresent invention is not limited to those embodiments.

Meaning of each reference numeral in the description below is asfollows.

-   -   1: Boiler,    -   2: NO_(x) removing apparatus,    -   3: Heat exchanger,    -   4: Dry dust-removing apparatus,    -   5: Wet desulfurization apparatus,    -   6: Wet dust-removing apparatus,    -   7: Moisture removing cooler,    -   8: CO₂ separation and liquefaction apparatus,    -   9: Discharge stack,    -   10: Oxygen generator,    -   11: Coal mill,    -   12: Burner,    -   13: After-gas port,    -   14: Recirculation gas supply pipe,    -   14 a: Primary gas pipe,    -   14 b: Secondary gas pipe,    -   14 c: After-gas pipe,    -   15: Nitrogen gas transfer pipe,    -   16: Oxygen supply pipe,    -   17: Recirculation exhaust gas moisture removing cooler,    -   19: Coal supply pipe,    -   20: Flue,    -   21, 21 a, 21 b: Recirculation fan,    -   22, 22 a, 22 b: Exhaust gas tapping port,    -   23: Oxygen and recirculation gas flow controller,    -   24: Oxygen analyzer, and    -   25: Oxygen controlling apparatus for oxidizer.

First Embodiment

FIG. 1 illustrates a configuration of an oxyfuel boiler system. Fuelcoal is supplied to a coal mill 11 via a coal transfer device and ispulverized to a particle size suitable for pulverized coal firing. Thepulverized coal (powdered coal) is supplied to a burner 12 through acoal supply pipe 19. The coal mill 11 is connected with a primary gaspipe 14 a which supplies recirculation exhaust gas to the coal mill 11.An oxygen supply pipe 16 is connected to a location along the coalsupply pipe 19, where oxygen is mixed as needed. Mixing of a properamount of oxygen with the recirculation exhaust gas in the coal supplypipe 19 has the effect of enhancing the ignition performance of the coalin the burner 12.

An oxygen generator 10 separates oxygen from air and supplies oxygen tothe coal supply pipe 19 and the like through the oxygen supply pipe 16.A great deal of nitrogen gas generated in separation of oxygen isdiffused from a discharge stack 9 by a nitrogen gas transfer pipe 15.

The burner 12 is provided with a secondary gas pipe 14 b, which is arecirculation gas supply pipe, and the oxygen supply pipe 16 isconnected to the secondary gas pipe 14 b. Also, the burner 12 ejects gasmixture of the oxygen supplied from the oxygen supply pipe 16 and therecirculation exhaust gas to a furnace. Further, the burner 12 alsoejects the pulverized coal supplied from the coal supply pipe 19 to thefurnace and forms a flame by the pulverized coal and the gas mixture.

An after-gas port 13 is arranged downstream of the burner 12, and therecirculation gas supply pipe 14 is connected to the after-gas port 13as well through an after-gas pipe 14 c. Also, the oxygen supply pipe 16is connected to the after-gas supply pipe 14 c as well. Further,independently of the burner 12, the after-gas port 13 also feeds the gasmixture of the oxygen and the recirculation gas to a boiler 1.

The after-gas port 13 has a function similar to that of the after-airport of an air fired boiler. That is, by properly regulating the flowamounts of the gas mixture and the oxygen concentration supplied to theburner 12 and the after-gas port 13, a burning zone of reductiveatmosphere is formed in the boiler 1, thus reducing the rate ofconversion of the nitrogen in the coal to NO_(x). Also, by the oxygencontained in the gas supplied from the after-gas port 13, a burning zoneof oxidative atmosphere is formed in the upper part of the boiler.Further, the jet exiting from the after-gas port 13 promotes gas mixingin the boiler and reduces unburned portion remaining in the exhaust gasof the boiler.

The flow rates of the recirculated exhaust gases supplied to the coalmill 11, burner 12, and after-gas port 13 can each be regulatedindependently by a flow rate regulator attached to the primary gas pipe14 a, secondary gas pipe 14 b, and after-gas pipe 14 c respectively.Further, the oxygen supply pipe 16 also has separate flow regulators inits branches for regulating the supply amount to the branches. Thesethree recirculation gas supply pipes 14 are connected with gas samplingpipes for measuring each oxygen concentration.

An oxygen controlling apparatus for oxidizer 25 comprises an oxygen andrecirculation gas flow controller 23 and an oxygen analyzer 24. Theoxygen analyzer 24 can always measure the oxygen concentration in thegas mixture in each pipe taken by a gas sampling pipe. The oxygen andrecirculation gas flow controller 23 can regulate the recirculation gasflow rate and the oxygen flow rate of the three recirculation gas supplypipes 14 independently. Also, the oxygen and recirculation gas flowcontroller 23 can automatically control the oxygen concentration usingthe oxygen concentration measured value from the oxygen analyzer 24 asan input signal. That is, with this controller, for the gas mixturesupplied to the boiler 1 from the coal supply pipe 19, secondary gaspipe 14 b, after-gas pipe 14 c, the distribution ratio of the oxygenfeeding amount and oxygen concentration can be set/regulatedindependently.

High temperature and high pressure steam generated in the boiler 1 issupplied to a steam turbine generating system and is converted toelectric power.

Exhaust gas generated in the boiler 1 is introduced to a flue 20 andsupplied to an NO_(x) removing apparatus 2, and the NO_(x) component inthe exhaust gas is reduced. In the flue 20, an exhaust gas treatmentsystem comprising a plurality of apparatuses treating the exhaust gas isprovided in addition to the NO_(x) removing apparatus 2. However, whenthe forming amount of NO_(x) can be sufficiently reduced by improvementof the combustion method and the like, the NO_(x) removing apparatus 2may be omitted. The exhaust gas exited from the NO_(x) removingapparatus 2 is supplied to a heat exchanger 3 and the temperature isreduced. Heat recovered from the exhaust gas by the heat exchanger 3 isgiven to the recirculation exhaust gas supplied similarly to the heatexchanger 3 for recirculating to the boiler 1 and inhibits deteriorationof thermal efficiency of the plant. The exhaust gas exited from the heatexchanger 3 is introduced to a dry dust-removing apparatus 4, and 95% ormore of the dust component is removed.

An exhaust gas tapping port 22 is disposed downstream of the drydust-removing apparatus 4. A part of the exhaust gas taken in from theexhaust gas tapping port 22 is induced to a recirculation gas supplypipe 14 by a recirculation fan 21 and is heated by the heat exchanger 3.The exhaust gas is thereafter supplied to the coal mill 11, burner 12and after-gas port 13 as described above.

95% or more of SO₂ of the exhaust gas not recirculated is removed by awet desulfurization apparatus 5. Then, the exhaust gas is removed of 98%or more of SO₃ component by a wet dust-removing apparatus 6, and themoisture content in the exhaust gas is substantially reduced by amoisture removing cooler 7. Further, in the present embodiment, withrespect to the removing apparatus for dust and sulfur compound in theexhaust gas such as the dry dust-removing apparatus, wet desulfurizationapparatus, and wet dust-removing apparatus, decision on necessity ornon-necessity of installation and alteration of specification of theremoval rate may be made at a designer's discretion. Also, an apparatuswith a similar function such as a dry desulfurization apparatus maybeinstalled as an alternative.

The CO₂ concentration of the exhaust gas exiting from the moistureremoving cooler 7 becomes approximately 90% or more. Therefore, a CO₂separation and liquefaction apparatus 8 can separate and liquefy CO₂ inthe exhaust gas easily. Also, the separated CO₂ may be supplied to auser through a pipe line and the like as high-pressure gas. The balancenot liquefied by the CO₂ separation and liquefaction apparatus 8 isdischarged as off-gas. Main components of the off-gas are nitrogen andoxygen, and minor components of NO_(x) and the like and some amount ofCO₂ are contained. The off-gas is mixed with a great deal of nitrogengenerated by the oxygen generator 10, and is diffused to the atmospherefrom the discharge stack 9.

Here, the basic principle of the oxyfuel boiler system will bedescribed. An ordinary coal firing boiler uses air as oxidizer gas,whereas the oxyfuel boiler uses the gas mixture in which major portionof the combustion exhaust gas is taken out from a location along theflue and is mixed thereafter with high purity oxygen generated by theoxygen generator for regulating the oxygen concentration as the oxidizergas. Thus, final exhaust gas flow rate discharged from the plant isreduced to approximately ¼th compared with other ordinary systems.Further, because the CO₂ concentration of the exhaust gas risesmassively, CO₂ can be separated and recovered from the exhaust gaseasily.

For CO₂ emission-free coal-firing oxyfuel boiler system, the maintechnical objectives are summarized as follows:

(1) To suppress reduction in power generation efficiency resulting fromenergy consumption in the oxygen generator and CO₂ separator.

(2) To establish a plant control method by which the plant can stablyrespond to various conditions such as starting, stopping and changingloads, and realize the stable operation by cooperation with peripheralfacilities (such as oxygen generator and a CO₂ separator).

(3) To achieve stable burning performance and suppress the formation oftrace harmful substance when burning coal with an oxidizer mixture gasof recirculation exhaust gas and oxygen.

(4) To prevent various problems arising from increased concentrations ofcomponents contained in the various exhaust gas resulting fromintroduction of a configuration in which a large amount of the exhaustgas is recirculated.

The present embodiment relates mainly to the item (3) above, and isobjected to solve the problems related to inhibit the forming amount ofNO_(x) which is the trace harmful substance. Below, the features of theoxyfuel boiler system and the problems related to NO_(x) reduction willbe described.

If CO₂ recovery from exhaust gas is the only object, burning coalexclusively with oxygen is an effective way. When coal is burnedexclusively with oxygen, the major components of the exhaust gas are CO₂and H₂O. Therefore, by separating and removing the H₂O from the exhaustgas, for example, by cooling the exhaust gas, CO₂ of a highconcentration can be readily collected. However, when coal is burnedwith oxygen only, the temperature of the burning flame is higher thanthose in air fired boiler system by 500 higher. Therefore, when thisoxygen firing is employed in a coal firing plant, expensive heatresistant steels need to be used for metal materials constituting theboiler. Another problem is that the oxidizer gas jet velocity in theburner is relatively low, thus making it difficult to forma stableflame. Also, the amount of the exhaust gas generated is less than ¼th ofthose in air firing, and therefore the velocity of the exhaust gasflowing through the heat transfer tube of the boiler is extremelyslower. Accordingly, the thermal transfer efficiency degrades and thethermal recovery becomes difficult.

To overcome above problems, the oxyfuel boiler system employs an exhaustgas recirculation system in which a large amount of the exhaust gas isrecirculated, mixed with oxygen, and then supplied to the boiler.Specifically, such system is designed so that the flow rate of theoxidizer gas supplied to the burner and the flow rate of the exhaust gasflowing through the boiler are not less than approximately 70% of theflow rate of the air in an air fired boiler. In this manner, highefficiency thermal recovery and electric power generation can be stablyachieved without greatly modifying a conventional air fired boilersystem.

In the oxyfuel boiler system with such basic configuration, the problemsrelated to NO_(x) reduction are as below.

In the oxyfuel boiler system, nitrogen (N₂) concentration in the exhaustgas is maintained extremely low. Therefore, thermal NO_(x) formed byreaction of N₂ and oxygen (O₂) in the high temperature zone in the upperpart of the boiler is suppressed greatly. As a result, the NO_(x)forming amount per unit supplied heat amount can be reduced comparedwith those in the air fired boiler. Accordingly, in order to furtherreduce the NO_(x) forming amount in the oxyfuel boiler system, theforming amount of fuel NO_(x) generated by oxidation of N portion incoal needs to be suppressed.

The reaction pathway that forms the fuel NO_(x) from N portion in coalis considered to be an oxidation reaction of ammonia (NH₃) and cyanogen(HCN) generated from the coal. The mechanism of the oxidation reactionof NH₃ and HCN is different in the reductive combustion zone withextremely low O₂ concentration and in the oxidative combustion zone withhigh O₂ concentration. OH radical mainly contributes to oxidation in thereductive combustion zone, whereas oxidation rests upon O₂ in theoxidative combustion zone. Accordingly, how these oxidation reactionrates are suppressed according to the atmosphere of the combustion zoneis a key point for the fuel NO_(x) reduction.

Also, as a factor affecting NO_(x) formation from NH₃ and HCN, thetemperature of the combustion zone is also important in addition to theO₂ concentration and the OH radical concentration described above. Inthe reductive combustion zone, as the combustion temperature becomeshigher, the forming rate of the fuel NO_(x) lowers. The reason isconsidered that the reaction in which the NO_(x) generated in thereductive combustion zone reacts with coal, NH₃, HCN and the like againand is reduced to N₂ is promoted in a field with higher temperature. Onthe other hand, in the oxidative combustion zone, as the combustiontemperature becomes lower, the forming rate of the fuel NO_(x) lowers.The reason is that, out of NH₃, HCN formed in the reductive combustionzone in the lower part of the boiler, those remaining until theoxidative combustion zone in the upper part of the boiler are oxidizedin this zone, and the N portion contained is converted to NO_(x) or N₂,where, when the combustion temperature of the oxidative combustion zoneis higher, conversion rate to NO_(x) rises, whereas when the combustiontemperature is lower, conversion rate to N₂ rises.

To summarize above, in order to suppress formation of the fuel NO_(x) inthe oxyfuel boiler system, it is necessary to lower the OH radicalconcentration in the reductive combustion zone to the most and to raisethe combustion temperature. Also, it is required to lower the O₂concentration in the oxidative combustion zone while controlling thecombustion temperature lower.

In this connection, in the present embodiment, the oxygen controllingapparatus for oxidizer 25 is provided with a function of regulating therecirculation gas amount supplied to the coal mill 11, which is a coalsupply apparatus, burner 12, and after-gas port 13 and setting the O₂concentration of the oxidizer gas independently.

In the air fired type coal firing boiler, the air excess ratio at theboiler outlet is made approximately 1.15, and the distribution ratio ofthe combustion air amount supplied to the burner and the combustion airamount supplied to the after-air port is made approximately 0.8:0.35.Therefore, NO_(x) and unburned portion can be reduced with a goodbalance. Regulating air excess ratio/distribution ratio of thecombustion air amount is equivalent with regulating oxygen excessratio/distribution ratio of the oxygen contained in the air. Therefore,in the oxyfuel boiler also, it is preferable to set the oxygen amountsupplied to the burner and the oxygen amount supplied to the after-gasport as well as the oxygen excess ratio at the furnace outlet using thecondition described above for the air firing case. That is, in thepresent embodiment, the total oxygen amount supplied to the boiler isset to approximately 1.15 times of the oxygen amount required forcomplete combustion of coal. Also, the distribution ratio of the oxygenamount supplied to the burner 12 and the oxygen amount supplied to theafter-gas port 13 is made approximately 0.8:0.35.

In addition, the oxygen controlling apparatus for oxidizer 25 also has afunction of regulating the recirculation gas amount supplied to theburner 12 and after-gas port 13. Furthermore, the oxygen controllingapparatus for oxidizer 25 sets the O₂ concentration of the oxidizer gasindependently. Specifically, the oxygen controlling apparatus foroxidizer 25 makes the concentration of the oxygen supplied to theafter-gas port lower than the concentration of the oxygen supplied tothe burner. With respect to the regulation target for the O₂concentration, following values are appropriate.

It is preferable to set the O₂ concentration of the oxidizer gas in thecoal supply pipe 19 and the secondary gas pipe 14 b connected to theburner between 32% and 36%. FIG. 5 illustrates a calculation result ofthe combustion temperature in the reductive combustion zone in theboiler 1 in accordance with the present embodiment. In order to obtainthe combustion temperature similar to that of the air firing, the O₂concentration of approximately 30% is needed. Also, it was found thatthe combustion temperature rose with the increase of the O₂concentration of the oxidizer gas. As described above, in the reductivecombustion zone, the forming rate of the fuel NO_(x) lowers with therise of the combustion temperature. Therefore, in order to obtain astable high temperature flame, O₂ concentration can be made 32% or more.

On the other hand, when the combustion temperature rises by 150° C. orhigher if compared with the time of the air firing, problems occur inthe heat resistance performance of the burner and boiler water tubematerial. Therefore, from this viewpoint, the O₂ concentration should be36% or less. Based on the above study, in order to reduce the formingamount of the fuel NO_(x), the O₂ concentration of the oxidizer gassupplied to the burner can be set in the range between approximately 32%and 36%.

Next, the O₂ concentration of the oxidizer gas in the after-gas pipe 14c connected to the after-gas port can be set between 26% and 28%. FIG. 6illustrates a calculation result of the combustion temperature in theoxidative combustion zone in the boiler 1 in accordance with the presentembodiment. By making the O₂ concentration 30%, the combustiontemperature approximately similar to that in the air firing can beobtained.

As described above, in the oxidative combustion zone, forming rate ofthe fuel NO_(x) lowers with the lowering of the combustion temperature.Therefore, in order to obtain a stable temperature lowering effect, theO₂ concentration can be made 28% or less.

On the other hand, when the combustion temperature lowers by 100° C. orhigher, there is a risk that the concentration of the unburned portionat the boiler outlet rises exceeding an allowable range. Therefore, theO₂ concentration should be made 26% or more. Based on the above study,in order to reduce the forming amount of the fuel NO_(x), the O₂concentration of the oxidizer gas supplied to the after-gas port can beset in the range between 26% and 28%.

Because the CO₂ emission-free oxyfuel boiler system in accordance withthe present embodiment can reduce the forming amount of the fuel NO_(x)generated in the combustion, NO_(x) reduction effect can be furtherenhanced. Also, the harmful component emitted to the atmosphere can bereduced substantially. Thus, the apparatuses related to NO_(x) removalin an exhaust gas treatment system can be substantially miniaturized andsimplified, and also the utilities cost of the ammonia solution and thelike required for their operation can be reduced.

Further, setting value of the appropriate O₂ concentration changes alsoaccording to the distribution ratio of the oxygen, oxygen excess ratio,boiler structure, kind of coal, and the like. Therefore, the settingvalue should be decided after each case is assessed, and the values ofthe appropriate O₂ concentration setting range described above does notlimit the scope of the present embodiment.

Second Embodiment

FIG. 2 illustrates a schematic diagram of the oxyfuel boiler system inaccordance with the present embodiment.

Because the present embodiment comprises many sections constituted ofapparatuses having the similar actions as those of the first embodiment,only the points different from the first embodiment will be describedbelow. The apparatuses not described below have the similar action andeffect as the first embodiment.

The point of the present embodiment different from the first embodimentis that the exhaust gas tapping port 22 is disposed downstream of themoisture removing cooler 7, and the recirculation exhaust gas removedwith the dust and moisture can be supplied to the boiler. The moistureremoving cooler 7 serves as a moisture removing apparatus. In theconfiguration in accordance with the first embodiment, the moisturedensity in the recirculation exhaust gas is approximately 30%. On theother hand, in the configuration in accordance with the presentembodiment, the moisture density in the recirculation exhaust gas can bemade 5% or less. Because the moisture density in the recirculationexhaust gas lowers, the moisture amount supplied from the burner alsodecreases. Therefore, OH radical concentration derived from the moisturein the reductive combustion zone greatly lowers. As described above,lowering of the OH radical concentration in the reductive combustionzone is effective in reduction of the fuel NO_(x).

Thus, with the configuration in accordance with the present embodiment,the NO_(x) reduction effect greater than that of the first embodimentcan be obtained.

Third Embodiment

FIG. 3 illustrates a schematic diagram of the oxyfuel boiler system inaccordance with the third embodiment.

Because the present embodiment comprises many sections constituted ofapparatuses having the similar actions as those of the first embodimentand the second embodiment, only the points different from the firstembodiment and the second embodiment will be described below. Theapparatuses not described below have the similar action and effect asthe said embodiments.

The configuration of the present embodiment is similar to the firstembodiment in that the exhaust gas tapping port 22 is disposeddownstream of the dry dust-removing apparatus 4 and upstream of theother exhaust gas treatment apparatuses. However, the configuration isdifferent in that the recirculation gas supply pipe 14 branches to twolines before entering the heat exchanger 3. Out of the recirculation gassupply pipes that branched, the first line is connected to the primarygas pipe 14 a and the secondary gas pipe 14 b through the heat exchanger3 after the moisture in the gas is removed through the recirculationexhaust gas moisture removing cooler 17. Out of the recirculation gassupply pipes that branched, the second line is connected to theafter-gas pipe 14 c through the heat exchanger 3 without removing themoisture. With the configuration in accordance with the presentembodiment, similar to the second embodiment, the OH radicalconcentration in the reductive combustion zone is substantially reduced.Also, the moisture in the recirculation exhaust gas supplied to theafter-gas port which does not affect the NO_(x) reduction is notremoved. Therefore, the thermal loss of the recirculation exhaust gassupplied to the after-gas port decreases, and the total capacity of themoisture removing cooler 7 and the recirculation exhaust gas moistureremoving cooler 17 can be reduced. Also, when the wet desulfurizationapparatus and the wet dust-removing apparatus are arranged like in thecase of the present embodiment, the gas amount flowing through theseapparatuses becomes less than ¼th of that of the second embodiment,therefore the apparatuses can be miniaturized.

Accordingly, with the present embodiment, the oxyfuel boiler systemhaving the NO_(x) reduction effect greater than that of the firstembodiment and with the improved thermal efficiency and with theminiaturized apparatuses when compared with the second embodiment can berealized.

Fourth Embodiment

FIG. 4 illustrates a schematic diagram of the oxyfuel boiler system inaccordance with the fourth embodiment.

Because the present embodiment comprises many sections constituted ofapparatuses having the similar actions as those of the third embodiment,only the points different from the third embodiment will be describedbelow. The apparatuses not described below have the similar action andeffect as the third embodiment.

The point of the present embodiment different from the third embodimentis that the exhaust gas tapping port for the recirculation gas isarranged in two positions of an exhaust gas tapping port 22 a and anexhaust gas tapping port 22 b. Similar to the third embodiment, theexhaust gas tapping port 22 a is disposed immediately after the drydust-removing apparatus. The exhaust gas taken in from the exhaust gastapping port 22 a is sent out to the boiler side by a recirculation fan21 a through the heat exchanger 3, and is supplied to the after-gas pipe14 c only. Also, similar to the second embodiment, the exhaust gastapping port 22 b is disposed immediately after the moisture removingcooler 7. The exhaust gas taken in from the exhaust gas tapping port 22b is supplied to the boiler side by a recirculation fan 21 b through theheat exchanger 3, and is supplied to the primary gas pipe 14 a and thesecondary gas pipe 14 b only.

With these configurations, similar to the third embodiment, the OHradical reduction effect in the reductive combustion zone can beobtained without arranging the recirculation exhaust gas moistureremoving cooler. That is, because it is not necessary to arrange themoisture removing apparatuses in two positions, layout of equipment andpiping is facilitated, therefore the apparatuses can be made more simpleand inexpensive.

According to the embodiments in accordance with the present invention,with configuration of a CO₂ emission-free oxyfuel boiler system, asubstantial reduction of the NO_(x) discharge can be realized by moresimple apparatuses, therefore spread of CO₂ emission-free powergeneration can be promoted contributing to suppressing global warming.

1. An oxyfuel boiler system comprising: an oxygen generator to separateoxygen from air; a coal mill to dry and pulverize coal; a burner to burnthe dried and pulverized coal with oxygen generated at the oxygengenerator; an after-gas port to which oxygen generated at the oxygengenerator is supplied; a boiler provided with the burner and theafter-gas port on its wall; a flue introducing combustion exhaust gasfrom the boiler to the outside; a recirculation gas supply pipe havingan exhaust gas tapping port disposed in the midway of the flue andsupplying recirculation exhaust gas to the coal mill, the burner, andthe after-gas port; and an oxygen supply pipe supplying oxygen from theoxygen generator to the burner and the after-gas port, wherein theexhaust gas tapping port is disposed downstream of a dry dust-removingapparatus arranged in the flue, and there is provided an oxygencontrolling apparatus for making a concentration of oxygen to besupplied to the after-gas port lower than that of oxygen to be suppliedto the burner.
 2. An oxyfuel boiler system according to claim 1, whereinthe exhaust gas tapping port is disposed downstream of the drydust-removing apparatus and downstream of a moisture removing cooler. 3.The oxyfuel boiler system according to claim 1, wherein the exhaust gastapping port is disposed downstream of the dry dust-removing apparatusand upstream of a first moisture removing cooler, and a second moistureremoving cooler is provided in the midway of the recirculation gassupply pipe.
 4. An oxyfuel boiler system comprising: an oxygen generatorto separate oxygen from air; a coal mill to dry and pulverize coal; aburner to burn the dried and pulverized coal with oxygen generated atthe oxygen generator; an after-gas port to which oxygen generated at theoxygen generator is supplied; a boiler provided with the burner and theafter-gas port on its wall; a flue introducing combustion exhaust gasfrom the boiler to the outside; recirculation gas supply pipes havingtwo exhaust gas tapping ports disposed in the midway of the flue andsupplying recirculation exhaust gas taken in from the respective exhaustgas tapping ports to the boiler; and an oxygen supply pipe supplyingoxygen from the oxygen generator to the burner and the after-gas port,wherein the oxyfuel boiler system has: the first exhaust gas tappingport arranged in the flue and disposed downstream of a dry dust-removingapparatus and the second exhaust gas tapping port arranged in the flueand disposed downstream of a moisture removing cooler; the recirculationgas supply pipe including a first line supplying exhaust gas taken infrom the first exhaust gas tapping port to the after-gas port and asecond line supplying exhaust gas taken in from the second exhaust gastapping port to the burner; and an oxygen controlling apparatus foroxidizer making a concentration of oxygen to be supplied to theafter-gas port lower than that of oxygen supplied to the burner.
 5. Amethod of controlling an oxyfuel boiler system, comprising: an oxygengenerator to separate oxygen from air; a coal mill to dry and pulverizecoal; a burner to burn the dried and pulverized coal with oxygengenerated at the oxygen generator; an after-gas port to which oxygengenerated at the oxygen generator is supplied; a boiler provided withthe burner and the after-gas port on its wall; a flue introducingcombustion exhaust gas from the boiler to the outside; a recirculationgas supply pipe having an exhaust gas tapping port disposed in themidway of the flue and supplying recirculation exhaust gas to the coalmill, the burner and the after-gas port; and an oxygen supply pipesupplying oxygen from the oxygen generator to pipes supplying exhaustgas to the coal mill, the burner and the after-gas port by therecirculation gas supply pipe, the method comprising the steps of:taking exhaust gas in from downstream of a dry dust-removing apparatusarranged in the flue; and making a concentration of oxygen to besupplied to the after-gas port lower than that of oxygen to be suppliedto the burner.